When a person starts a posting on a message board, or forum, or Facebook, that others are able to comment on, that original posting and the comments on it are called a thread. A thread hijacking occurs when one or more individuals commenting on the original posting, go off topic, creating a separate conversation. This is rude, and bad internet etiquette. If people want to discuss a different topic, they should start their own thread.

original posting: I'm so excited that my cat has learned to use the toilet with City Kitty! The training seemed like it took forever, but it's so worth it to not have to deal with a litter box any more! Has anyone else done this?

comment 1: Ugh! I just started training my cat, but he can't seem to move beyond the orange stage. How'd you get through that?

comment 2: I'm training my dog at PetCo classes. She's very smart. She learns the commands faster than the other dogs.

comment 3: I'm training my dog in PetCo classes too. They have great trainers. They're patient with the dogs and the people.

Comments 2 and 3 are off topic and are hijacking this thread. The person who made comment 2 should have started their own thread. The person who made comment 3 is guilty of continuing the hijacking.